The invention relates to novel pentanedioic acid derivatives. In particular, it relates to novel pentanedioic acid derivatives of formula I which are useful to inhibit the formation of serum cholesterol. The novel compounds exhibit this utility by virtue of their ability to inhibit the activity of .beta.-hydroxy-.beta.-methylglutaryl coenzyme A (HMG CoA) and thus inhibit the formation of serum cholesterol. HMG CoA is a substance which controls the rate at which cholesterol is synthesized in mammalian liver (one of the two principal in vivo sources of serum cholesterol). Thus the compounds of the instant invention are useful to inhibit sterol biosynthisis in individuals predisposed to familial type hypercholesterolemia. The significance of such compounds is widely recognized, e.g. Breslow et al. Biochim. Biophys. Acta, 398,10(1975); Betheridge et al., Brit. Med. J., 4,500 (1975); and Brown et al., J. Biol. Chem. 249, 7306(1974).